[spareparts]

the barb fitting is part of the idle air system, as the engine warms up, it allows more air into the intake trac to lean out the mixture, it should have one of the barbs connected to a hose taht runs to the front half of the engine, the other barb is left open.

Checking the sensors is a good starting point, its the ones with the tan/blue wire.
Go ahead and put a water pump in it, its regular mantance on a Merc, its the best starting point for figuring out whats wrong.